The present invention relates to dielectric materials, and more particularly relates to electronically tunable dielectric composite materials provided in the form of relatively thick films. Methods such as screen printing and spray deposition are used to prepare thick film composite materials comprising an electronically tunable phase such as barium strontium titanates and at least one non-tunable phase.
Phased array antennas comprise a large number of elements that emit phased signals to form a radio beam. The radio signal can be electronically steered by the active manipulation of the relative phasing of the individual antenna elements. This electronic beam steering concept applies to both the transmitter and the receiver. Phased array antennas are advantageous in comparison with their mechanical counterparts with respect to their speed, accuracy, and reliability. For example, the replacement of gimbal scanned antennas by their electronically scanned counterparts provides more rapid and accurate target identification. Complex tracking exercises can also be performed rapidly and accurately with a phased array antenna system.
Future communications will require wideband communications using frequency-hopping techniques, so that a large amount of digital data can be transferred over a bandwidth. A critical component for these applications is a low cost, fast-acting tunable filter. Digital data could be distributed or encoded over a band of frequencies in a sequence determined by controlling circuitry of the tunable filter. This would allow several users to transmit and receive over a common range of frequencies.
Common varactors used today are silicon and GaAs based. The performance of these varactors is defined by the capacitance ratio, Cmax/Cmin, frequency range and Q (1/tan xcex4) at the specified frequency range. The Q factors for these semiconductor varactor for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q of these varactors degrades rapidly. In fact, at 10 GHz the Q factors for these varactors are usually only about 10.
Barium strontium titanates (BaTiO3xe2x80x94SrTiO3), also referred to as BSTO, is used for its high dielectric constant (200-6,000) and large change in dielectric constant with applied voltage (25-75 percent with a field of 2 Volts/micron). Dielectric materials including barium strontium titanates are disclosed in U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-MgOxe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Material-BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidesxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectronically Graded Multilayer Ferroelectric Compositesxe2x80x9d; U.S. Pat. No. 5,635,433 to Sengupta entitled xe2x80x9cCeramic Ferroelectric Composite Material BSTO-ZnOxe2x80x9d; U.S. Pat. No. 6,074,971 to Chiu et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materials with Enhanced Electronic Properties BSTO-Mg Based Compound-Rare Earth Oxidexe2x80x9d; U.S. application Ser. No. 09/594,837 filed Jun. 15, 2000, entitled xe2x80x9cElectronically Tunable Ceramic Materials Including Tunable Dielectric and Metal Silicate Phasesxe2x80x9d; U.S. application Ser. No. 09/768,690 filed Jan. 24, 2001 entitled xe2x80x9cElectronically Tunable, Low-Loss Ceramic Materials Including a Tunable Dielectric Phase and Multiple Metal Oxide Phasesxe2x80x9d; and U.S. Provisional Application Ser. No. 60/295,046 filed Jun. 1, 2001 entitled xe2x80x9cTunable Dielectric Compositions Including Low Loss Glass Fritsxe2x80x9d. These patents and applications are incorporated herein by reference.
The idea of a voltage tunable dielectric has been proposed for use in antenna applications, as set forth by Richard W. Babbit et al. in a publication entitled xe2x80x9cPlanar Microwave Electro-Optic Phase Shifters,xe2x80x9d Microwave Journal, Volume 35 (6), (June 1992). This publication concludes that a need exists for additional research to be conducted in the materials art to yield materials having more desirable electronic properties.
Although various types of tunable dielectric composite materials are known, prior art methods are not conducive to the use of these types of materials in phase shifter applications and phased array antennas at frequencies above 10 GHz which require direct integration into a lithographic antenna design. Also, the dielectric constants of the bulk materials render them impractical or impossible for use in varactors which have capacitances of less than 2 pF, and in co-planar waveguide phase shifters where the impedance of the device includes the low dielectric constant substrate material. For these applications it is desirable to have films from 2-25 microns in thickness and to provide a method for film deposition onto low cost low dielectric constant microwave substrates.
A need has developed for the fabrication of thick film materials having improved electronic properties which may have ideal properties with use in, for example, planar varactors, tunable filters, tunable oscillators, tunable vertical single layer and multi-layer capacitors, co-planar and transmission line phase shifters, and phased array antennas. There is also a need for multi-layered thick films in the multi-layer capacitor industry in order to allow for the creation of tunable vertical capacitors that have large capacitance and are extremely low cost. These capacitors as well as planar capacitors form the basis of high power tunable filters and resonators. Additionally, a need exists for providing a low cost method of impedance matching using multi-layer compositions that have superior electronic properties.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.
The present invention provides for the fabrication of electronically tunable dielectric composite thick film materials which reduce or eliminate polishing or pick-and-place techniques for insertion into devices and/or antennas. The thick films of the present invention are particularly useful in integrated antenna designs. The present invention permits printing of phase shifters and antenna elements onto the same substrate, providing a tunable dielectric monolithilic integrated circuit. According to an embodiment of the invention, thick film voltage tunable dielectrics have enhanced electronic properties and are superior to thin film configurations because the thick films have lower loss tangents and are not dependant on substrate materials. Single crystal substrates are not necessary. Therefore, devices incorporating the present thick film composite materials are relatively inexpensive. For example, an entire antenna can be printed in a single pattern by a one-step process.
The electronically tunable thick film composite materials described in preferred embodiments of the present invention can be used to fabricate low cost phased array antennas due to the elimination of low noise amplifiers and other electronics. Also, the deposition of the material onto low cost substrates substantially reduces the cost of the phase shifting elements.
Varactors fabricated from the electronically tunable thick film composites of the present invention can be used independently, or can be integrated into low cost tunable filters. These varactors and filters can be used at numerous frequency ranges, including frequencies above the L-band, in many different commercial and other applications.
An aspect of this invention to provide electronically tunable dielectric composite thick films suitable for use in applications such as phased array antenna systems and the like. The thick film composites of the present invention demonstrate increased tunabilities, reduced loss tangents, lower dielectric constants, the ability to operate at higher frequency due to reduced thicknesses in comparison with bulk ceramics, and the ability to reduce the overall size of the phase shifting components. The present invention also reduces or eliminates the need for machining components. A unique aspect of this invention relative to conventional thin film composites lies in the fact that the thick film composites demonstrate much lower loss tangents, the ability to use low cost microwave substrate materials, and much less expensive deposition costs. The present invention permits printing of phase shifters and antenna elements onto the same substrate in a single step process that does require dicing, bonding and pick-and-place techniques.
Another aspect of the present invention is to provide a method of making electronically tunable dielectric composite thick films. The composites preferably comprise from about 30 to about 99.9 weight percent of an electronically tunable dielectric phase such as barium strontium titanates, and from about 0.1 to about 70 weight percent of at least one additional phase. In addition to the production of single thick film layers, the present invention provides for multilayered deposition with or without interdigitated electrodes using either singular or multiple compositions. The ceramic particle size is preferably controlled to achieve optimal electronic and microwave properties.
A further aspect of the present invention is to provide an electronically tunable thick film dielectric material having a thickness of from about 2 to about 25 microns, preferably from about 3 to about 15 microns.
A further aspect of the present invention is to provide a device comprising a ceramic substrate and an electronically tunable thick film dielectric material deposited on the substrate.
Another aspect of the present invention is to control particle sizes of electronically tunable and non-tunable phases of the dielectric composite in order to control the loss tangents, dielectric constants and tunabilities of the thick films.
A further aspect of the present invention is to the fabricate thick film materials which have sought after properties in, for example, phase array antennas as well as other devices such as tunable filters. The sought after properties include a moderate dielectric constant, low loss tangent and high tunability.